During the drilling of a wellbore, various fluids are used in the well for a variety of functions. The fluids may be circulated through a drill pipe and drill bit into the wellbore and then may subsequently flow upward through wellbore to the surface. During this circulation, drilling fluids act to lubricate and cool rotary drill bits, to prevent blowouts by providing hydrostatic pressure to balance any high-pressure formation fluids that may suddenly enter the wellbore, and to remove cuttings from the wellbore.
Wellbore fluids may also be used to provide sufficient hydrostatic pressure in the well to prevent the influx and efflux of formation fluids and wellbore fluids, respectively. When the pore pressure (the pressure in the formation pore space provided by the formation fluids) exceeds the pressure in the open wellbore, the formation fluids tend to flow from the formation into the open wellbore. Therefore, the pressure in the open wellbore may be maintained at a higher pressure than the pore pressure. While it is highly advantageous to maintain the wellbore pressures above the pore pressure, on the other hand, if the pressure exerted by the wellbore fluids exceeds the fracture resistance of the formation, a formation fracture and thus induced mud losses may occur. Further, with a formation fracture, when the wellbore fluid in the annulus flows into the fracture, the loss of wellbore fluid may cause the hydrostatic pressure in the wellbore to decrease, which may in turn also allow formation fluids to enter the wellbore. As a result, the formation fracture pressure may define an upper limit for allowable wellbore pressure in an open wellbore while the pore pressure defines a lower limit. Therefore, a major constraint on well design and selection of drilling fluids is the balance between varying pore pressures and formation fracture pressures or fracture gradients though the depth of the well.
One common step during the construction of the wellbore involves placing a pipe string, e.g., casing, into the wellbore. Well casings of various sizes may be used, depending upon depth, desired hole size, and geological formations encountered. The casing serves several functions, including providing structural support to the wellbore to prevent the formation walls from caving into the wellbore. The casing may, in some instances, be stabilized and bonded in position within the wellbore. However, because drilling fluids are generally not settable (i.e. they don't develop compressive strength or create a solid bond with casing and formation surfaces), a portion of the drilling fluid may be removed from the wellbore so that the casings may be set in place by a primary cementing operation. Primary cementing operations may fill at least a portion of the annular space between the casing and the formation wall with a hydraulic cement composition. The cement composition may then be allowed to solidify in the annular space, thereby forming an annular sheath of cement. The cement barrier is desirably impermeable, such that it will prevent the migration of fluid between zones or formations previously penetrated by the wellbore.
Cementing operations may also include use of cement during remediation of lost circulation or zonal isolation. Lost circulation is a recurring drilling problem, characterized by loss of drilling mud into downhole formations. However, other fluids, besides “drilling fluid” can potentially be lost, including completion, drill-in, production fluid, etc. Lost circulation can occur naturally in formations that are fractured, highly permeable, porous, cavernous, or vugular. Lost circulation may also result from induced pressure during drilling. Specifically, induced mud losses may occur when the mud weight, required for well control and to maintain a stable wellbore, exceeds the fracture resistance of the formations. A particularly challenging situation arises in depleted reservoirs, in which the drop in pore pressure effectively weakens a wellbore through permeable, potentially hydrocarbon-bearing rock formation, but neighboring or inter-bedded low permeability rocks, such as shales, maintain their pore pressure. This can make the drilling of certain depleted zones impossible because the mud weight required to support the shale exceeds the fracture resistance of the sands and silts. Another unintentional method by which lost circulation can result is through the inability to remove low and high gravity solids from fluids. Without being able to remove such solids, the fluid density can increase, thereby increasing the hole pressure, and if such hole pressure exceeds the formation fracture pressure, fractures and fluid loss can result. In such operations, a cement slurry may be forced under pressure to areas of lost integrity in the annulus to seal off those areas.
Other situations arise in which isolation of certain zones within a formation may be beneficial. For example, one method to increase the production of a well is to perforate the well in a number of different locations, either in the same hydrocarbon bearing zone or in different hydrocarbon bearing zones, and thereby increase the flow of hydrocarbons into the well. The problem associated with producing from a well in this manner relates to the control of the flow of fluids from the well and to the management of the reservoir. For example, in a well producing from a number of separate zones (or from laterals in a multilateral well) in which one zone has a higher pressure than another zone, the higher pressure zone may disembogue into the lower pressure zone rather than to the surface. Similarly, in a horizontal well that extends through a single zone, perforations near the “heel” of the well, i.e., nearer the surface, may begin to produce water before those perforations near the “toe” of the well. The production of water near the heel reduces the overall production from the well.
Before cementing operations commence, engineers determine the volume of cement (commonly with the help of a caliper log) to be placed in the wellbore and the physical properties of both the slurry and the set cement needed, including density and viscosity.